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The structure of the [Fe(o-phen)3]2+ complex cation in ferroin
3D model (JSmol)
ECHA InfoCard 100.035.145
Molar mass 596.27 g/mol
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Ferroin is the chemical compound with the formula [Fe(o-phen)3]SO4, where o-phen is an abbreviation for 1,10-phenanthroline, a bidentate ligand. The term "ferroin" is used loosely and includes salts of other anions such as chloride.

Redox indicator[edit]

Phenanthroline Fe(II) (Redox indicator)
E0= 1.06 V
Reduced. Oxidized

This coordination compound is used as an indicator in analytical chemistry.[1] The active ingredient is the [Fe(o-phen)3]2+ ion, which is a chromophore that can be oxidized to the ferric derivative [Fe(o-phen)3]3+. The potential for this redox change is +1.06 volts in 1 M H2SO4. It is a popular redox indicator for making oscillatory Belousov–Zhabotinsky reactions visible.

Ferroin is suitable as a redox indicator, as the color change is reversible, very pronounced and rapid, and the ferroin solution is stable up to 60 °C. It is the main indicator used in cerimetry.[2]

Nitroferroin, the complex of iron(II) with 5-nitro-1,10-phenanthroline, has transition potential of +1.25 volts. It is more stable than ferroin, but in sulfuric acid with Ce4+ ion it requires significant excess of the titrant. It is however useful for titration in perchloric acid or nitric acid solution, where cerium redox potential is higher.[2]

The redox potential of the iron-phenanthroline complex can be varied between +0.84 V and +1.10 V by position and number of methyl groups on the phenanthroline core.[2]


Ferroin sulfate may be prepared by combining phenanthroline to ferrous sulfate in water.

3 phen + Fe2+ → [Fe(phen)3]2+

The iron is low spin and octahedral with D3 symmetry. The intense color of this ferrous complex arises from a metal-to-ligand charge-transfer transition.


  1. ^ Harris, D. C. Quantitative Chemical Analysis (4th ed.). New York, NY: W. H. Freeman. ISBN 0-7167-2508-8.
  2. ^ a b c https://books.google.cz/books?id=_Ro3Fqtz4xgC&pg=PA289&lpg=PA289&dq=alternative+to+ferroin&source=bl&ots=kXHliTI_a_&sig=JeKKZEvsW23OLYTr_qIipxI1rcM&hl=en&sa=X&redir_esc=y#v=onepage&q=alternative%20to%20ferroin&f=false